Honeycomb core composite sandwich structures find widespread use in the aerospace industry as panel components in various aerospace structures. The honeycomb core composites are formed from a lay-up of prepreg skin plies encompassing a honeycomb core, the latter typically having beveled edges. The prepreg plies may be fabrics, tapes, or non-wovens that have been pre-impregnated with a thermosetting, thermoplastic or other polymeric resin. The fabrics used to form the prepregs are woven fabrics, formed primarily or entirely of high modulus, reinforcing fibers in the form of continuous filament tows. Curing of the lay-up is carried out in a high temperature, high pressure environment, typically in an autoclave.
The technical requirements of aerospace end uses generally dictate that the prepregs and prepreg components meet a rigid set of chemical, physical, and mechanical specifications including overall prepreg basis weight, fiber modulus, and resin flow rate. The basis weight of the prepreg and the high strength properties of the fibers and the resin, in combination with the strength properties of the honeycomb core component, impart high strength-to-weight, and high stiffniess-to-weight ratios to the fmal composite structure. In addition, the flow rate characteristics of the resin and the high pressures used to cure the composite, minimize porosity, i.e., the inclusion of voids and through holes, that might impair strength, the desired impervious nature, and/or surface smoothness of the final honeycomb panel sandwich structure.
Even though honeycomb core composite panels have long been used in the aerospace industry, manufacture of these structures is still plagued by high reject scrap levels, generating substantial quantities of unusable scrap and impacting negatively on manufacturing economics. Partial collapse of the honeycomb core during curing of the composite, known in the industry as "core crush", is a particularly common reason for rejection of cured panels. Core crush is typically observed in the beveled edge or chamfer region of the honeycomb structural part.
Substantial effort and research extending over many years have been directed to the core crush problem. For example, U.S. Pat. No. 5,685,940 to Hopkins discloses an improved tiedown method to produce or prevent core crush and ply wrinkling in honeycomb sandwich structures. A scrim-supported barrier film is placed between the fiber-reinforced resin composite laminates and honeycomb core to prevent resin flow from the prepreg into the honeycomb core. A tiedown ply between the core and the barrier film is used to reduce slippage of the barrier film relative to the core during curing. In addition, a film adhesive having a curing temperature lower than that of the laminate resin is placed between the tiedown plies just outside the net trim line. During the curing process, cured film adhesive bonds the tiedown plies to one another before the curing of the prepreg laminates, thus strengthening the tiedown and reducing core crush. The Hopkins patent also discusses other methods and structural modifications which have been proposed for minimizing or eliminating the core crush. Nevertheless, core crush remains a significant problem in the industry.